THE SOLUBILITY OF NB-TA-ZR-HF-W IN GRANITIC MELTS WITH LI AND LI- CONSTRAINTS FOR MINERALIZATION IN RARE-METAL GRANITES AND PEGMATITES(F )

Tantalum mineralization is associated with granites and pegmatites tha t invariably are rich in lithium. This in part is due to the extreme f ractionation that is necessary for melts to reach saturation in either a Ta or an Li phase. However, there may also be chemical interactions in melts that explain this association. Experiments, therefore, were conducted to determine the effect of lithium on the solubility of tant alite (MnTa2O6) in water-saturated granitic melts. Other important ele ments in rare metal granites include Nb, Zr, Hf, and Mi. Consequently the effect of Li on the solubilities of columbite (MnNb2O6), zircon (Z rSiO4), hafnon (HfSiO4), and hubnerite (MnWO4) in granitic melts was a lso investigated. The haplogranitic melt compositions used in these ex periments have constant mole percent Si and Al, and the Al/(Li + Na K) ratio was 1.0 for all experiments, i.e., the lithium content was th e only compositional variable. Columbite and tantalite solubilities in the granitic liquids increase by a factor of similar to 2 to 3 with t he addition of 2 wt percent Li2O to haplogranite at 750 degrees to 1,0 35 degrees C, 2 kbars, and water-saturated conditions. The solubility data were extrapolated to a lower temperature, more reasonable for col umbite-tantalite crystallization, then compared to compositions repres entative of evolved liquids from which rare metal granites and pegmati tes crystallize. It is estimated that at 600 degrees C these melt comp ositions could be saturated in columbite but not tantalite. However, t antalite saturation is predicted if the melts contained less Li and F. Therefore, the genesis of tantalum mineralization may be explained by Ta being retained in the melt because of high Li F concentrations. Ta ntalite crystallization is delayed until an Li +/- F +/- P mineral cry stallizes, which lowers tantalite solubility and results in a general association of Ta with Li in mineralized granitic rocks. In contrast t o columbite and tantalite, the solubility of wolframite is not affecte d by the lithium content of the melt (with up to 3.8 wt % Li2O in star ting glasses) and the solubilities of zircon and hafnon decrease with increasing Li content of the melt. One possible interpretation of the hubnerite solubility data is that the melt contains dominantly W6+, wh ich behaves as a network former. The decrease in zircon-hafnon solubil ity with Li may reflect the lower field strength of Zr4+ and Hf4+, com pared to Nb5+ and Ta5+, i.e., Zr and Hf are less able to compete with Li for nonbridging oxygens, whereas Nb and Ta are more able to compete . It is also significant that the solubility of hafnon is greater than that of zircon, similar to the higher solubility of tantalite compare d to columbite. This can explain why Zr/Hf and Nb/Ta ratios both decre ase with fractionation.